1. Field of the Invention
Process for Producing Aluminum Oxide Beads The invention relates to a process for producing aluminum oxide beads, in which an acid aluminum oxide sol or an acid aluminum oxide suspension having a viscosity of 10 to 500 mPa.s is produced and converted into droplets, the droplets are coagulated in an aqueous ammonia solution, and the gel beads thereby formed are aged, washed, dried and calcined. Aluminum oxide beads can be used as, for example, adsorbents in chromatography or as catalysts or catalyst carriers.
When used in a fixed bed, the bead shape of the catalyst permits a very even packing of the catalyst inside the reactor. In addition, bead-shaped carriers have a low tendency to formation of unwelcome channels. In a moving catalyst bed (moving bed), the good flow properties of round particles also have an advantageous effect.
2. Description of the Related Art
A process for producing micro-beads in the form of uranium oxide is known from FR-A-2 387 076. The droplets, which are laterally blown with ammonia gas, are formed by a vibrated nozzle plate. The diameter of the micro-beads is approximately 80 .mu.m.
A known process for producing aluminum oxide beads is described in U.S. Pat. No. 2,620,314, whereby a hydrosol is made from aluminum chips, aluminum trichloride and water that is then mixed with an aqueous hexamethylene tetramine solution and dripped into a hot oil bath. The gel beads are aged for at least 10 hours in hot oil, then washed, dried and calcined.
Further known processes based on this principle of solidification of aluminum oxide hydrosols in forming columns filled with hot oil are described in U.S. Pat. Nos. 2,774,743, 3,096,295, 3,600,129, 3,714,071, 3,887,492, 3,919,117, 3,943,070, 3,972,990, 3,979,334, 4,250,058 DE-C 27 42 709, DE-C 29 42 768 and DE-C 29 43 599.
In U.S. Pat. No. 4,116,882, a process is described whereby an aluminum oxide filter cake obtained by hydrolysis from aluminum alkoxides is peptized with the aid of a dilute acid, and the resultant slurry dripped into a forming column in which the top phase comprises naphtha and the bottom phase a 10% ammonia solution. To reduce the surface tension between the hydrocarbon phase and the aqueous phase, a non-ionic, surface-active agent is added to the ammonia solution. The gel beads are aged in aqueous ammonia solution, dried and calcined.
A process is known from DE-A 28 12 875 whereby a slurry is initially formed from a microcrystalline boehmite/pseudo-boehmite intermediate product by addition of an acid.
This slurry is then dripped into a hydrocarbon/ammonia solution forming column, and the gel beads thereby obtained are dried and calcined. The shaped elements have a specific surface area of 90 m.sup.2 /g to 120 m.sup.2 /g and a bulk density of 0.42 g/cm.sup.3 to 0.51 g/cm.sup.3.
DE-C 32 12 249 describes a process for producing bead-shaped aluminum oxide whereby a stable hydrosol is obtained by dispersion of a mixture of boehmite and pseudo-boehmite in an aqueous acid in the presence of urea. This sol is then shaped by dripping it into a hydrocarbon/ammonia solution forming column. Here too, a surface-active agent is added to the ammonia solution. In addition, the possibility is demonstrated of influencing substantial bead properties such as porosity, bulk density and breaking strength by using sol additives in the form of hydrocarbons and suitable emulsifiers.
DE-A 33 46 044 describes a process for carrier production on an aluminum oxide basis, in which a suspension or aqueous dispersion is produced by stirring boehmite into an aqueous acid with the addition of an aluminum oxide obtained from boehmite by tempering. This suspension or dispersion is then mixed with an organic phase and an emulsifying agent, and the emulsion obtained ("oil in water" type) is shaped in the known manner in a two-phase column. The pore volume and bulk density can be adjusted within certain limits by varying the weight proportions of hydrocarbon and calcined aluminum oxide used in the sol.
U.S. Pat. No. 3,558,508 describes a process for producing aluminum oxide beads by pouring an acid dispersion of aluminum oxide hydrate into a forming column and dripping with a mixture of mineral oil and carbon tetrachloride. The oil/CCl.sub.4 mix is saturated with gaseous ammonia, thereby achieving solidification of the sol droplets as they sink inside the forming column.
DE-A 32 42 293 describes a process for producing beads having a diameter of 0.01 to 2 mm. Small sol droplets are obtained here by atomizing an acid aluminum oxide sol with an inert gas or inert liquid. The droplets can be coagulated either in a forming column of the "hydrocarbon/aqueous ammonia" type or in hot air (spray drying).
U.S. Pat. No. 4,198,318 describes a process in which aluminum oxide particles with substantially spherical form are produced by dripping hydrosols of low viscosity into an aqueous ammonia solution. The acid hydrosol is dripped into the ammonia phase from a drop height of 0.5 to 2 cm. To facilitate immersion of the droplets into the coagulation medium, a non-ionic surface-active agent is admixed to the ammonia solution. In this process, in which the use of hydrocarbons as the shaping media is dispensed with, it is obviously of especial importance that the very short drop height is very precisely optimized and maintained, since the form of the gel beads can only be adjusted using this drop height.
High throughputs are not however obtained in this process variant, because a maximum of 5 drops per second can be attained per nozzle. In addition, there is always the risk with so short a drop height that the nozzles can be clogged by rising ammonia vapors.
DE-C 24 59 445 describes a process for producing mutually identical, bead-shaped fuel particles by conversion of a liquid jet of solutions containing uranium or thorium and flowing out of one or more nozzles and made to vibrate, in a quantity of 3000 droplets per minute, whereby the droplets first pass through an ammonia-free drop distance before immersion in the ammonia solution, said distance being dimensioned such that the droplets have just taken on their bead shape and then immediately pass through a drop distance filled by ammonia gas in order to stabilize this bead shape, with the ammonia gas being introduced into this drop distance such that in addition to an ammonia gas flow opposite to the droplet fall direction a horizontal transverse flow component of the ammonia gas is ensured through the droplet intervals, with this drop distance being dimensioned such that the bead-shaped droplets harden sufficiently before immersion into the ammonia solution.